No abstract

Marine plastic debris floating on the ocean surface is a major environmental problem. However, its distribution in the ocean is poorly mapped, and most of the plastic waste estimated to have entered the ocean from land is unaccounted for. Better understanding of how plastic debris is transported from coastal and marine sources is crucial to quantify and close the global inventory of marine plastics, which in turn represents critical information for mitigation or policy strategies. At the same time, plastic is a unique tracer that provides an opportunity to learn more about the physics and dynamics of our ocean across multiple scales, from the Ekman convergence in basin-scale gyres to individual waves in the surfzone. In this review, we comprehensively discuss what is known about the different processes that govern the transport of floating marine plastic debris in both the open ocean and the coastal zones, based on the published literature and referring to insights from neighbouring fields such as oil spill dispersion, marine safety recovery, plankton connectivity, and others. We discuss how measurements of marine plastics (both in situ and in the laboratory), remote sensing, and numerical simulations can elucidate these processes and their interactions across spatio-temporal scales. Environ. Res. Lett. 15 (2020) 023003 E van Sebille et al Environ. Res. Lett. 15 (2020) 023003 E van Sebille et al References Acha E M, Mianzan H W, Iribarne O, Gagliardini D A, Lasta C and Daleo P 2003 The role of the Rı́o de la Plata bottom salinity front in accumulating debris Mar. Pollut. Bull. 46 197-202 Acha E M, Piola A, Iribarne O and Mianzan H 2015 Ecological Processes at Marine Fronts: Oases in the Ocean (Berlin: Springer) Aliani S and Molcard A 2003 Hitch-hiking on floating marine debris: macrobenthic species in the Western Mediterranean Sea Hydrobiologia 503 59-67 Allen J 1985 Principles of Physical Sedimentology (Berlin: Springer) Alpers W 1985 Theory of radar imaging of internal waves Nature 314 245-7 Alsina J M and Cáceres I 2011 Sediment suspension events in the inner surf and swash zone. Measurements in large-scale and high-energy wave conditions Coast. Eng. 58

Estimates suggest that one hundred times more plastic enters the ocean annually than is found at the sea surface (Van Sebille et al., 2015). It is still unknown where the plastic goes and how much resides in the other ocean reservoirs, including the sea floor (Courtene-

Abstract. The fate of (micro)plastic particles in the open ocean is controlled by biological and physical processes. Here, we model the effects of biofouling on the subsurface vertical distribution of spherical, virtual plastic particles with radii of 0.01–1 mm. The biological specifications include the attachment, growth and loss of algae on particles. The physical specifications include four vertical velocity terms: advection, wind-driven mixing, tidally induced mixing and the sinking velocity of the biofouled particle. We track 10 000 particles for 1 year in three different regions with distinct biological and physical properties: the low-productivity region of the North Pacific Subtropical Gyre, the high-productivity region of the equatorial Pacific and the high mixing region of the Southern Ocean. The growth of biofilm mass in the euphotic zone and loss of mass below the euphotic zone result in the oscillatory behaviour of particles, where the larger (0.1–1.0 mm) particles have much shorter average oscillation lengths (<10 d; 90th percentile) than the smaller (0.01–0.1 mm) particles (up to 130 d; 90th percentile). A subsurface maximum particle concentration occurs just below the mixed-layer depth (around 30 m) in the equatorial Pacific, which is most pronounced for larger particles (0.1–1.0 mm). This occurs because particles become neutrally buoyant when the processes affecting the settling velocity of a particle and the seawater's vertical movement are in equilibrium. Seasonal effects in the subtropical gyre result in particles sinking below the mixed-layer depth only during spring blooms but otherwise remaining within the mixed layer. The strong winds and deepest average mixed-layer depth in the Southern Ocean (400 m) result in the deepest redistribution of particles (>5000 m). Our results show that the vertical movement of particles is mainly affected by physical (wind-induced mixing) processes within the mixed-layer and biological (biofilm) dynamics below the mixed layer. Furthermore, positively buoyant particles with radii of 0.01–1.0 mm can sink far below the euphotic zone and mixed layer in regions with high near-surface mixing or high biological activity. This work can easily be coupled to other models to simulate open-ocean biofouling dynamics, in order to reach a better understanding of where ocean (micro)plastic ends up.

It is well known that ecological processes such as population regulation and natural enemy interactions potentially occur over a range of spatial scales, and there is a substantial body of literature developing theoretical understanding of the interplay between these processes. However, there are comparatively few studies quantifying the long-term effects of spatial scaling in natural ecosystems. A key challenge is that trophic complexity in real-world biological communities quickly obscures the signal from a focal process. Seagrass meadows provide an excellent opportunity in this respect: in many instances, seagrasses effectively form extensive natural monocultures, in which hypotheses about endogenous dynamics can be formulated and tested. We present amongst the longest unbroken, spatially explict time series of seagrass abundance published to date. Data include annual measures of shoot density, total above-ground abundance, and associated epiphyte cover from five Zostera marina meadows distributed around the Isles of Scilly, UK, from 1996 to 2011. We explore empirical patterns at the local and metapopulation scale using standard time series analysis and develop a simple population dynamic model, testing the hypothesis that both local and metapopulation scale feedback processes are important. We find little evidence of an interaction between scales in seagrass dynamics but that both scales contribute approximately equally to observed local epiphyte abundance. By quantifying the long-term dynamics of seagrass-epiphyte interactions we show how measures of density and extent are both important in establishing baseline information relevant to predicting responses to environmental change and developing management plans. We hope that this study complements existing mechanistic studies of physiology, genetics and productivity in seagrass, whilst highlighting the potential of seagrass as a model ecosystem. More generally, this study provides a rare opportunity to test some of the predictions of ecological theory in a natural ecosystem of global conservation and economic value.

Determining whether the Atlantic Meridional Overturning Circulation (AMOC)'s transport is in decline is challenging due to the short duration of continuous observations. To estimate how many years are needed to detect a decline, we conduct a simulation study using synthetic data that mimics an AMOC time series. The time series' characteristics are reproduced using the trend, variance, and autocorrelation coefficient of the AMOC strength at 26.5 • N from 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) models under the RCP8.5 future scenario, and from RAPID observations (2004-2018). Our results suggest that the 14-year RAPID length has just entered the lower limits of the trend's "detection window" based on synthetic data generated using CMIP5 trends and variability (14-42 years; median = 24 years), but twice the length is required for detectability based on RAPID variability (29-67 years; median = 43 years). The annual RAPID trend is currently not statistically significant (−0.11 Sv yr −1 , p > 0.05). Plain Language Summary There are ongoing discussions in the scientific community about whether the Atlantic Meridional Overturning Circulation transport is slowing down. This is of interest due to the importance of this circulation in transporting heat from the tropics to the northern latitudes. A consensus about its decline is hard to reach due to the limited direct observational data available; with the longest continuous data being 14 years long from 2004 to 2018. We therefore conduct a simulation experiment to examine how many years of data are required to detect a decline in the circulation. We create simulations of the North Atlantic transport based on statistical properties from 20 general circulation models with future climate change projections (until 2100) and from the RAPID array observations (since 2004). Our results demonstrate that the length of data we currently have from observations has just entered the "detection window" of 14-42 years (based on model simulations). However, the RAPID observations do not currently exhibit a statistically significant trend.

Abstract. The fate of (micro)plastic particles in the open ocean is controlled by physical and biological processes. Here, we model the effects of biofouling on the subsurface vertical distribution of spherical, virtual plastic particles with radii of 0.01–1 mm. For the physics, four vertical velocity terms are included: advection, wind-driven mixing, tidally induced mixing, and the sinking velocity of the biofouled particle. For the biology, we simulate the attachment, growth and loss of algae on particles. We track 10,000 particles for one year in three different regions with distinct biological and physical properties: the low productivity region of the North Pacific Subtropical Gyre, the high productivity region of the Equatorial Pacific and the high mixing region of the Southern Ocean. The growth of biofilm mass in the euphotic zone and loss of mass below the euphotic zone result in the oscillatory behaviour of particles, where the larger (0.1–1.0 mm) particles have much shorter average oscillation lengths (< 10 days; 90th percentile) than the smaller (0.01–0.1 mm) particles (up to 130 days; 90th percentile). A subsurface maximum concentration occurs just below the mixed layer depth (around 30 m) in the Equatorial Pacific, which is most pronounced for larger particles (0.1–1.0 mm). This occurs since particles become neutrally buoyant when the processes affecting the settling velocity of the particle and the motion of the ocean are in equilibrium. Seasonal effects in the subtropical gyre result in particles sinking below the mixed layer depth only during spring blooms, but otherwise remaining within the mixed layer. The strong winds and deepest average mixed layer depth in the Southern Ocean (400 m) result in the deepest redistribution of particles (> 5000 m). Our results show that the vertical movement of particles is mainly affected by physical (wind-induced mixing) processes within the mixed layer and biological (biofilm) dynamics below the mixed layer. Furthermore, positively buoyant particles with radii of 0.01–1.0 mm can sink far below the euphotic zone and mixed layer in regions with high near-surface mixing or high biological activity. This work can easily be coupled to other models to simulate open-ocean biofouling dynamics, in order to reach a better understanding of where ocean (micro)plastic ends up.

&lt;p&gt;Microplastic debris ending up at the sea surface has become a known major environmental issue. However, how microplastic particles move and when they sink in the ocean remains largely unknown. Here, we model microplastic subject to biofouling (algal growth on a substrate) to estimate sinking timescales and the time to reach the depth where particles stops sinking. We combine NEMO-MEDUSA 2.0 output, that represents hydrodynamic and biological properties of seawater, with a particle-tracking framework. Different sizes and densities of particles (for different types of plastic) are simulated, showing that the global distribution of sinking timescales is largely size-dependent as opposed to density-dependent. The smallest particles we simulate (0.1 &amp;#956;m) start sinking almost immediately around the globe and their trajectories produce the longest time to reach their first sinking depth (almost 40 days as a global median). In oligotrophic subtropical gyres with low algal concentrations, particles between 1 mm and 10 &amp;#956;m do not sink within the 90-day simulation time. This suggests that in addition to the comparatively well-known physical processes, biological processes might also contribute to the accumulation of floating plastic (of 1 mm to 10 &amp;#956;m) in subtropical gyres. Particles of 1 &amp;#956;m in the gyres start sinking largely due to vertical advection, whereas 0.1 &amp;#956;m particles sink both due to biofouling and advection. The qualitative impacts of seasonality on sinking timescales are small, however, localised sooner sinking due to spring algal blooms is seen. This study maps processes that affect the sinking of virtual microplastic globally, which could ultimately impact the ocean plastic budget.&lt;/p&gt;